13C NMR spectroscopy of carbon nanohorns

We report the results of a carbon-13 nuclear magnetic resonance spectroscopic investigation of the structure of carbon nanohorn aggregates (CNHs). The results show that CNHs consist of two components, characterized by different chemical shifts and spin lattice relaxation (T-1) behavior. The first component has a chemical shift of 124 ppm and displays rapid spin-lattice relaxation behavior and is assigned to the nanotubelike horns on the particles' surfaces. The second component has a chemical shift of 116 ppm and much slower spin-lattice relaxation behavior and is assigned to the graphitelike part of the CNH aggregrate. The results of integrated peak area measurements indicate a 1:2 ratio of nanohorns to the graphitelike substrate. The absence of a clear Korringa behavior for the temperature dependence of T-1 and the lack of a Knight shift ruled out any metallic behavior and indicated instead behavior characteristic of semiconductor materials with paramagnetic centers due to structural defects providing an effective relaxation mechanism in the nanohorn domains. We also observed an anomalous change in T-1 near 17 K in the nanohorn domains suggesting the development of an antiferromagnetic correlation between localized electron spins.